racket/gui cocoa: make 'border handling more flexible

When buttons are hidden and/or disabled so that there's a change to
the only button that is visible, enabled, and has the 'border style,
then make it the default.

pkgs/gui-pkgs/gui-lib/mred/private/wx/cocoa/button.rkt

@@ -42,7 +42,7 @@
         [button-type #f])
   (init-field [event-type 'button])
   (inherit get-cocoa get-cocoa-window init-font
-           register-as-child)
+           register-as-child get-wx-window)
 
   (define button-cocoa
     (let ([cocoa 
@@ -154,7 +154,22 @@
   (define default-button? (memq 'border style))
   (define/override (show-children)
     (when default-button?
-      (tellv (get-cocoa-window) setDefaultButtonCell: (tell button-cocoa cell))))
+      (send (get-wx-window) add-possible-default this)))
+  (define/override (hide-children)
+    (when default-button?
+      (send (get-wx-window) remove-possible-default this)))
+
+  (define/override (enable-window on?)
+    (super enable-window on?)
+    (when default-button?
+      (send (get-wx-window) queue-default-button-check)))
+
+  (define/public (be-default) ; called by frame, only when the button is shown
+    ;; return #t to indicate succes, #f to let some other button take over
+    (and (tell #:type _BOOL button-cocoa isEnabled)
+         (begin
+           (tellv (get-cocoa-window) setDefaultButtonCell: (tell button-cocoa cell))
+           #t)))
 
   (tellv button-cocoa setTarget: button-cocoa)
   (tellv button-cocoa setAction: #:type _SEL (selector clicked:))

pkgs/gui-pkgs/gui-lib/mred/private/wx/cocoa/frame.rkt

@@ -596,6 +596,28 @@
 
     (define/public (set-icon bm1 [bm2 #f] [mode 'both]) (void)) ;; FIXME
 
+    (define default-buttons (make-hasheq))
+    (define checking-default? #f)
+    (define/public (add-possible-default button)
+      (hash-set! default-buttons button #t)
+      (queue-default-button-check))
+    (define/public (remove-possible-default button)
+      (hash-remove! default-buttons button)
+      (queue-default-button-check))
+    (define/public (queue-default-button-check)
+      (when (atomically
+             (if checking-default?
+                 #f
+                 (begin
+                   (set! checking-default? #t)
+                   #t)))
+        (queue-window-event 
+         this
+         (lambda ()
+           (set! checking-default? #f)
+           (for/or ([button (in-hash-keys default-buttons)])
+             (send button be-default))))))
+
     (define/override (call-pre-on-event w e)
       (pre-on-event w e))
     (define/override (call-pre-on-char w e)